A fossil-fired steam generator generates superheated steam with the aid of the heat generated as the result of the combustion of fossil fuels. Fossil-fired steam generators are mainly used in steam power plants, which predominantly serve the purpose of generating electricity, the steam generated being supplied to a steam turbine.
Along similar lines to the various pressure stages in a steam turbine, fossil-fired steam generators likewise encompass a plurality of pressure stages with different thermal states of the respective water-steam mixture contained therein. In the first (high) pressure stage, the flow medium runs on its flow path first through the economizers, which use residual heat to pre-heat the flow medium, and subsequently through various stages of evaporator and super heater heating surfaces. The flow medium is evaporated in the evaporator, and then any residual moisture is separated off in a separating device and the remaining steam is further heated in the super heater. Then the superheated steam flows into the high pressure section of the steam turbine, is released there and supplied to the subsequent pressure stage of the steam generator, where it is superheated again (intermediate super heater) and supplied to the next pressure section of the steam turbine.
Due to various external influences, the heat output transmitted to the super heaters may vary considerably. Therefore, it is frequently necessary to regulate the superheating temperature. This is usually achieved mostly by an injection of feedwater upstream or downstream of individual super heater heating surfaces to cool them, that is, an overflow line branches off from the main flow of the flow medium and leads to injection valves that are disposed there accordingly. In such cases, the injection is usually controlled by means of a characteristic value characteristic of the temperature deviations from a predetermined nominal temperature value at the super heater outlet.
Modern power plants are expected not only to achieve high degrees of efficiency, but also a mode of operation that is as flexible as possible. In addition to short start-up times and fast load change rates, this also involves the possibility to compensate for frequency disturbances in the electricity grid. To meet these expectations, the power plant must be in the position to provide additional power of, for example, 5% and more within a few seconds.
Such changes in the power provided by a power plant unit in a time frame of seconds are only possible with the aid of a co-ordinated interaction of the steam generator and the steam turbine. The contribution that the fossil-fired steam generator can make thereto is the use of its storage accumulators, that is, of the steam accumulator but also of the fuel accumulator, in addition to rapid changes in the controlling variables of feedwater, injection water, fuel and air.
This can ensue, for example, by the opening of partly throttled turbine valves of the steam turbine or of what is known as a step valve, by means of which the steam pressure is lowered upstream of the steam turbine. As a result, steam is released from the steam accumulator of the upstream fossil-fired steam generator and is supplied to the steam turbine. This measure allows an increase in power to be achieved within a few seconds.
A permanent throttling of the turbine valves to maintain a reserve always leads, however, to a loss in the degree of effectiveness such that for an economic mode of operation the degree of throttling should be kept as low as is absolutely necessary. Moreover, some designs of fossil-fired steam generators, such as, for example, forced-flow steam generators, sometimes have a considerably lower storage volume than, for example, natural circulation steam generators. In the method described in the aforementioned, the difference in the size of the accumulator affects the performance when there are changes in the power of the power plant block.